e-MERLIN 21cm constraints on the mass-loss rates of OB stars in Cyg OB2

Abstract

We present e-MERLIN 21 cm (L-band) observations of single luminous OB stars in the Cygnus OB2 association, from the COBRaS Legacy programme. The radio observations potentially offer the most straightforward, least model-dependent, determinations of mass-loss rates, and can be used to help resolve current discrepancies in mass-loss rates via clumped and structured hot star winds. We report here that the 21 cm flux densities of O3 to O6 supergiant and giant stars are less than ~ 70 microJy. These fluxes may be translated to `smooth' wind mass-loss upper limits of ~ 4.4 - 4.8 x 10^(-6) M_sol/yr for O3 supergiants and < 2.9 x 10^(-6) M_sol/yr for B0 to B1 supergiants. The first ever resolved 21 cm detections of the hypergiant (and LBV candidate) Cyg OB2 #12 are discussed; for multiple observations separated by 14 days, we detect a ~ 69% increase in its flux density. Our constraints on the upper limits for the mass-loss rates of evolved OB stars in Cyg OB2 support the model that the inner wind region close to the stellar surface (where Halpha forms) is more clumped than the very extended geometric region sampled by our radio observations.

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• Upper Mass-Loss Limits and Clumping in the Intermediate and Outer Wind Regions of OB stars
• On the binary nature of massive blue hypergiants: high-resolution X-ray spectroscopy suggests that Cyg OB2 12 is a colliding wind binary
• Sensitivity of Polarization to Grain Shape: I. Convex Shapes
• Variable millimetre radiation from the colliding-wind binary Cyg OB2 #8A
• Estimation of the flux at 1450MHz of OB stars for FAST and SKA